Amplifier gas tube



Sept. 23, 1952 W. M. WEBSTER, JR

AMPLIFIER GAS TUBE Filed Nov. 22, 1950 s/a/v/rz. 500/305 7 lllllll ||||||1|l|||||l||||||||| 4 ORNEY F'atented Sept. 23, 1 952 AMPLIFIER GAS TUBE 5 William.Merle"Webster, Jr., Princeton, N. 1., assignor to Radio Corporation of America, a. corporation of Delaware ApplicationNovember 22, 1950,-Serial No. 197,014

'1 This invention relates" to 'improveinents'in gas tubes which are suitable for use as amplifiers in that they have 'continuousf'grid control. More particularly it"relate's to amplifier gas tubesof a particular kind having very highvalues of transconductanc'e and'of 'a'node current and extremely l'owvalues of outputimpedance which are described in the'co-p'ending application'a'ssigned to the same assignee as the'present application, Serial No. 185,745, filed September 20, 1950 (RCA 33802). Tubes of this'kind have ofiered very great advantages over the tubes which precededthe'm. However, they also have certain limitations which will best be'understood by first reviewing theirbasic operating principles. In any such tube thereare separate discharge paths for the loadcurrent and theionizing current. During operation the energizing potential required for drawing load current, from a main cathode to' a main anode, is always below thevalue' required to produce ionization; However, a separate ionizingi electron discharge, an auxiliary discharge, -is energized with'a higherpotential to assure ionization. .It ionizes the gaseous filling oflthe tube creating a' conductive plasma; by converting neutral gas" atoms into positive ions and detached, free; negative electrons. The plasma surrounds the main cathode and fills the load current discharge path. As a result theelectron. space charge surrounding the cathode is neutralized'by positive ions and the plasma .efiectively acts as a low impedance. conductor'connected between the cathode and the anode. I

Since the auxiliary discharge is energized with a steady direct potential the plasma'de'nsity will remain constant. Moreover, neither the voltage variations of the signal'applied to the control electr'ode, in its location in the load current discharge path, nor the corresponding variations of low-:velocity loadcur-rent' discharge are effective toichan'ge the constant density of the plasma. Thus the plasma influences only the magnitude of 'fthe direct current component of the load current.-''".', .1 i. The load current in a tuberof this kind may. easily be hundreds'of times greater than" the auxiliary discharge current upon which it depends.

Because of the fact that the load current electron discharge from the main cathode to the main anode is incapableof producing ionization, acontrol grid located between these electrodes isable; toretaincontrol even "thoughit is "entirely im nersed-inrthe plasma: Thiswas 'agreat ad van'ce over previous gas tubes in whichthe-p of the ionizing discharge always coincided-with thatof the load current discharge, and-therefore extended with itfthroughthe: control grid o'p'er'iing'(s)," with the result that the grid w as d'e prived ofany further'abilityto control the anode current 'once the gaseous filling bf the tube be"- car'ne ionized. 'The mechanics of why the negatively biased control grid' can vary the load current' is". .that as the input signal varies the instantaneousnegative potential of the' 'grida positive ion sheath which surrounds it varies' in thickness and 'this varies the .1 cross-sectional areas of the columns ofplasma 'whicheiitend thruiits opening'sI Y T Ti Two limitations of tubesof this kind have been: that the grid-voltage versus plate-current characteristics have'usually been somewhat non-- linear and thatthe attainable transconductances are rather limited .due to the fact that the grid must be capable of directly controlling. 'a. very large magnitude current. 1:; llj' Itis the object of the,presentinvention'tozdevise a gasamplifier' tube of the. kind in question with a grid-voltage versus plate current chara'ce teristicof improved linearity; n i L It is a further object of the present invention to provide gas tube amplifiers of the ;k indyi.in question of even greater valuesrof transconducc a 1 These objects have been obtainedby making certain modifications of the, tubes mentioned above- According tothe principal modiflcation of., he present invention the control grid is located a fie i p t wh rei iiiable 2. trol the ma nit e the auxili y elec ron discharge rather than to directly control that of the load current electron, discharge- The purpose in so doing is to-indirectlyefiectco r51 of thel'oad currentbyvaryingfthe density 6' the plasma. Since the rate ai whic iheiinsjity. the plasma can. diminish, is; aTfunc t ion bi di'fi. ization time, and therefore, is limited, thisltype of control results in lowering the upp liinit of the frequency range of operation; ever, more linear 'eg/ ip characteristics result 'iiam this kind of control while at the same time'the at: tainable upper frequency 1imi f is still high enough for many applicationssfich "as audiouse In addition, since direct control of the relatively small ionizing current'results'in indirect contil of .the' very large. load current the attainable transconductances benefit from a multiplicatmfi efiect' analogousito that'..i'obtain'ed' in secondary emission electron multiplier tubes. Extremely large transconductances, such as transconductances of the order of one or more mhos, have been attained.

In the drawing:

Figure 1 represents a longitudinal sectional view of an illustrative embodiment. The section is taken along line I-I of Figure 2 and in a plane which is parallel to the axes of the cylindrical cathodes;

Figure 2 represents another longitudinal sectional view of this embodiment, this section being taken along the line 22 of Figure 1 and in a plane perpendicular to that of the section of Figure 1; and

Figur 3 is a schematic circuit diagram of an audio amplifier system using the gas tube shown in Figs. 1 and 2.

The gas tube I shown in Figs. 1 and 2 comprises a gas-tight envelope II which, in this illustrative embodiment, is of substantially rectangular shape. The load current in this tube originates at a main source of electrons, consisting of two indirectly heated cathodes l2, I3, and

flows to a main anode I4 and an accelerating grid I5. An auxiliary cathode I6 serves as a source of electrons for the ionizing discharge. According to the present invention the ionizing electron current is amplitude modulated. To this end the auxiliary cathode I6 is arranged with a number of other electrodes to constitute what may becalled an auxiliary triode. These elements are a control grid I1, which in the example herein is constructed in the same manner as semi-cylindrical half of a bird cage grid; a slotted anode I8; and a shield I 9. In the operation of this tube the anode I8 draws electrons from the auxiliary cathode I6 and those which emerge from its slot 20 are then attracted into the region of the main load-current paths of the tube.

The anode I4 and the shield I9 are supported on rods 2| and 22, respectively, which are sealed through the opposite ends of the envelope II to serve as terminal pins and support rods. The grid I1 is supported on a pair of wires 23, 24 the latter of which extends through the bottom of envelope I l to serve as a terminal pin. The main and auxiliary cathodes are supported by wires which are fused through the top of envelope II, such aswires 25, 26, 2'! and 28, respectively, which appearin- Figure 1. In accordance with the usual practice, one of each of these pairs of wires serves as a terminal pin for the cathode sleeve as well as for one side of the heater thereof, while the other serves as the terminal pin for the opposite side" of the heater. The slotted anode I8 as well as the accelerating grid I and a screen grid 28 are supported between the top and bottom of env'elope II as shown in Figure 1. Suitable ones of the support wires used for this purpose extend through the bottom of the envelope to provide a respective terminal pin for each of these elemerits.

In order not to include unnecessary detail in the drawing, and because the cathodes may be of any of a number of suitable types known in the art, none of the cathodes is shown in section. In general, indirectly heated types of cathodes (particularly for main cathodes) are to be preferred for a number of reasons, e. g., their greater rigidity (than filamentary cathodes) and the resultant freedom from microphonics; the reduced likelihood of hum when A.-C. heater current is used; and the more copious emission of electrons.

Each of the improved tubes shown herein may be processed in any of a number of ways well known in the art to provide a gaseous filling G within its envelope prior to scaling off. Any suitable gas or mixture of gases may be utilized. The gas pressure for a particular pick-up will be in accordance with its specific electrode geometry and spacings and must be such as to favor the formation of a self-sustaining ionizing discharge. A number of gas tubes of the kinds employed in the present pick-ups have been found to operate satisfactorily with a filling of helium at a pressure of approximately 750 microns. However, as is well known other gases and other pressures may be used, c. g., gas pressures which lie within the range between approximately microns and several millimeters of mercury.

Figure 3 shows a circuit which is suitable for using the gas tube shown in Figures 1 and 2. In the operation of the tube the potential of the slotted anode I8 is positive with respect to auxiliary cathode I6 but not sufliciently so to produce ionization between them.

Some of the electrons which are attracted from the auxiliary cathode I6 toward the slotted anode I8 will drift through the slot 20 and toward a group of electrodes including those which carry the useful load currents. Since the auxiliary triode is operated with such a low anode potential it will behave like a vacuum tube rather than a gas tube. That is to say it will be possifble to use its grid to efiect plate current variations in a substantially linear fashion over a usefully large range. Referring again to Figure 3, two separate direct current potential sources are shown, a low potential source 30 and a high potential source 3 I. The source 30 is used for maintaining the small potential difference between the main source of electrons, the pair of cathodes I2, I3, and the load current collecting electrodes. the accelerating screen I5 and the main anode I 4. The source 30 may consist of a number of series-parallel-connected 1 v. dry cells, a stor age battery, or any similar fairly-high current source. This is because it must serve not merely for establishing a bias but also for delivering to the utilization device 31 a signal-bearing current having an average value of the order of one or several amperes. Conversely the source 3I should be capable of providing a relatively much higher potential but it need not be capable of providing large continuous currents. It serves primarily as the energizing source for the ionizing discharge and in addition it provides a number of bias potentials. The source 3| comprises a battery 32, providing a potential well in excess of that required to ionize the gaseous fill ing of the tube I8, and a voltage divider 33 having a number of movable taps. A ground connection 34 is at such a point in source 3I that it provides a negative bias potential for the grid I! as well as a number of adjustable positive potentials. The grid I1 is biased for class A operation and an input signal from a source 35 is algebraically added to the bias by some appropriate means such as the transformer 39 shown herein.

The entire group of low current carrying elec trodes, i. e., the two main cathodes I2, I3, the anode I4, and the accelerating grid I5 may be considered as a composite final accelerating anode for the ionizing discharge. Accordingly, this entire group of electrodes is polarizedat a relatively high potential as shown in Figure 3. The screen 29 is polarized at a potential intermediate that ofaslotted anode and the auxiliary. cathode. The reason for selection of such potentials will be understood from the description which follows.

For proper operation of the tube II] it is desirable to prevent or inhibit any tendency of positive ions to drift from the right hand region of the tube back through the slot 20 into the interelectrode spaces of the auxiliary triode where their presence would tend to impair its proper operation. Accordingly, means are provided herein for trapping positive ions in two low potential troughs from which it will be difiicult for them to escape so as to be free to move toward the triode section. A first low potential trough may be established just beyond the slotted anode 18 by properly polarizing the screen 29. In the operation of this device, this screen is maintained at a potential above that of the auxiliary cathode IE but below that of the slotted anode l8. As a result the electrons which, as explained above, will emerge from the slot 20 at velocities too low to produce ionization, will be retarded by the screen 29 (but not stopped) so as to emerge from its right hand side at even lower velocities. The next element in the path of these electrons is the accelerating grid [5. In the normal operation of tube Hi, this electrode is maintained at a potential somewhat above the ionizing potential of its gaseous filling G. In this arrangement it is to be expected that at least some of the electrons approaching the accelerating grid l will have ionizing collisions slightly before they reach it and that for this reason positive ions will be present to the left of this element (as it appears in the drawing). However, any leftward movement of these ions will be limited as most of them will be attracted into and trapped within the low potential trough of the screen 29. Of course, the inertia of some of these ions will be sufficient so as to tend to carry'them all the way up the left slope of the potential trough. However, a large percentage of these will lose much of their inertia in collision with gas atoms. Moreover, most of those which do get up the left slope will collide with solid portions of the slotted anode l8 so that only a very few will ever pass through its slot 20. The slower ions will oscillate leftward and rightward through the screen 29 thereby remaining trapped in the trough which it establishes and eventually they will be collected by this element and neutralized. Most of the ionizing collisions will occur in a second low potential trough to the right of the accelerating grid IS in which, therefore, a dense plasma will be established. It is in this region that the main source of thermally emitted electrons is located. This source in the present example is a pair of indirectly heated cathodes 12, I3. The main anode I4 is mounted to the right of these cathodes. It may be polarized at the same potential as the accelerating grid l5 while the two cathodes l2, [3 are polarized at a potential a few volts below it, i. e., below the common potential for these two load-current-collecting elements. It is in this way that the second low potential trough is established between the accelerating grid [5 and the main anode l4 and it is in this trough that the greatest number of positive ions will be trapped. During the operation of the tube the electron discharge current which issues from the slot is modulated in accordance with the signal potential-variations of the grid I1.

6 In thepreferred embodiment shown herein certain electrodes are common to both theloadcur-g rent and the auxiliary current discharge: paths.

' These are the electrodes [21-15 which actiasa envelope containing a gaseous filling; said .enl-

velope further containing .a main cathode andxa main anode in cooperative spaced. relationship; at opposite ends of a load current path therein; 'a space charge limited cathode and an electrode for receiving electrons therefrom positioned. with-, in the envelope and surrounded by said gaseous filling at opposite ends of an auxiliary discharge path which does not coincide with said load current path but a portion of which is in close proximity thereto; terminals over which asourceof ionizing potential maybe connected between said auxiliary cathode and said receiving electrode for producing an ionizing discharge to provide'a conductive plasma between said main cathode and anode; and a control grid adjacent said auxiliary cathode for controlling the ionizing discharge and thereby the plasma density.

2. A gas tube as in claiml which furthercomprises an electron-transparent electrode positioned in a region athwart said auxiliary discharge path on the far side of said grid from said auxiliary cathode; and a terminal over which a source of potential may be connected to said transparent electrode for trapping positive ions in said region to prevent them from entering the space between the auxiliary cathode and the control grid.

3. A gas tube comprising; a sealed envelope containing a gaseous filling; a group of load circuit electrodes within the envelope and including at least one main cathode and a main anode having, respectively, electron-emitting and electron receiving surfaces which define opposite ends of a predetermined load current path; means including a space charge limited auxiliary cathode and an electrode for collecting electrons therefrom surrounded by said gaseous filling and positioned in spaced-apart relationship for producing an ionizing discharge of electrons along a path within said envelope which does not coincide for all of its length with all of said load current path but includes a portion in such close proximity thereto, for example, coincidental therewith or crosswise or adjacent thereto, that plasma produced by said ionizing discharge bathes the main cathode and extends over all of said current path; a control grid adjacent said auxiliary cathode for controlling the ionizing electron current; an electron-transparent electrode positioned in a region athwart said ionizing discharge path on the far side of said grid from said auxiliary cathode; and a terminal over which a source of ionizing potential may be connected to said transparent electrode for trapping positive ions in said region and preventing them from reaching the auxiliary cathode .and the space between it and said control grid.

aeue'so .4. Ages. tubeasin claim 3 which further comprises a first accelerating electrode .between said control grid and said transparent electrode for drawing electrons fromthe space charge of ,the auxiliary "cathodeto project them along the path of the :ionizing discharge and thru the transparent. electrode, and a post-accelerating electrode on thefar side of said transparent electrode ifromv the first accelerating electrode .for acceleratingthe last-mentioned electrons .to project :them with ionizing velocities over said portlonlof the path of the ionizing discharge which isiinsproximity to the load current-path "5.:A :gas tube comprising; a sealed envelope containing a gaseous filling; a group of loadecircuit electrodes within the envelope andzincluding atileast one main cathode and a. main anode having, respectively, electron-emittingland electron receiving surfaces which define. opposite ends, of a predetermined loadecurrent path; means including'an auxiliary cathode and an electrode for receiving electrons therefrom surrounded by said gaseous filling and positioned in spaced relationship .for producing an ionizing discharge of electrons along a path within said envelope which does not coincide for all of its length with all of said load current path but includes aportion in suchclose proximity thereto, for example, coincidental therewith or crosswise or adjacentthereto, that plasma produced by said ionizing .discharge bathes the main cathode and extends over all of said load current path; a control grid adjacent said auxiliary cathode for controlling the ionizingelectron current; a slotted anode on the far side of said grid from said auxiliary cathode for drawing electrons through the former from 8 the latter; and an. electron-transparent: ,electrod for-producing axlow potential trough athwart': the path of theiauxiliary discharge in a region he.- tween the slotted anode-and said portionlofzthe ionizing: discharge path which is1in. CIOSQPIOXP imity to the load current path.

6. A gas tube. as in claimfi which furthercomprises an electron transparent post-accelerating electrode between said first mentioned transparent electrode andsaid portion of the discharge path in proximity to the load current path.

7. An electron discharge device inculdingan envelope containing a gaseous atmosphere, a main cathode and a main anode within said onvelope defining a load current path, a space charge limited auxiliary cathode outside said path for supplying electrons for ionizing the space between said main cathode and anode, said main cathode and anode being adapted to have a potential applied therebetween less than the ionizing potential, a grid adjacent said auxiliary cathode between it and said path, and an apertured electrode between said grid and path through which electrode electrons from said auxiliary cathode may be projected, said grid and apertured electrode being adapted to be maintained at less than ionizing potential with respect to said auxiliary cathode, and an accelerating electrode between said apertured electrode and said path for accelerating electrons from said auxiliary cathode to ionizing potential in a region near to said main cathode.

WILLIAM MERLE WEBSTER, JR.

No references cited. 

